1. Field of the Invention
The present invention relates to an electromagnetic wave shielding sheet, and particularly to an electromagnetic wave shielding sheet suitable for use in electronic parts, circuit boards, flexible printed circuit boards (FPCs) and the like.
2. Description of the Related Art
With miniaturization of electronic equipment requiring higher density assembly, high integration, and lighter electronic equipment, damage by electromagnetic waves from electronic parts and/or electronic equipment has become a problem. In order to avoid the problem, various preventive measures have been taken. In order to shield an object from electromagnetic waves, the object is shielded with a metallic material. Examples of metallic materials used for this purpose include metal plates, wire gauzes, metallic leaves or foils (hereinafter referred to as metallic leaf), platings, deposited metals and metal-containing paints. They are properly used as necessary for the end application intended. Electromagnetic wave shielding properties generally become better as the volume resistivity of a shielding material is lower, and its thickness is greater. However, the prior art methods for shielding are limited by size, price, performance, processability and the like according to its application.
However, because of the present rapid progress of electronic technologies to make electronic parts smaller and lighter in weight, electromagnetic wave shielding materials used in these electronic parts must be made thinner, for example, by changing from a metal plate to a wire gauze or metallic leaf. For example, in a flexible printed circuit board, an electromagnetic wave shielding material is laminated on one side or both sides of the circuit board to shield it from electromagnetic noise generated from other electronic parts. Since there is a limit to built-in volume, the electromagnetic wave shield is required, for example, not to impair the flexibility of the board and electromagnetic wave shielding performance. However, only metallic leaf and wire gauze can be used in the existing circumstances. So, the flexibility of the shielding materials remains unsatisfactory under such circumstances.
By way of example, metallic leaf include stainless steel leaf, aluminum leaf, copper leaf and gold leaf. A sheet-like leaf 50 .mu.m or smaller in thickness may be produced. However, it is desired to replace the metallic leaf with other materials because the weight of a part coated with metallic leaf becomes heavy due to its great specific gravity. Moreover, metallic leaf has poor in processability, and its cost is comparatively high. On the other hand, wire gauze is porous compared with metallic leaf. Also, the exposed surface area of a sheet composed of the wire gauze is greater than the metallic leaf. Therefore, wire gauze has the advantage of enabling it to lighten the whole weight without impairing the electromagnetic wave shielding performance. However, a monofilament thinner than 50 .mu.m in diameter cannot be obtained. So, it is difficult to obtain a wire gauze having a thickness thinner than 100 .mu.m even with plain weave processing. Therefore, wire gauze presents problems with respect to thickness and flexibility of the shielding element.